More complex than we are told

“On a very simple level, your weight depends on the number of calories you consume, how many of those calories you store, and how many you burn up,” explains a publication from Harvard Medical School. “But each of these factors is influenced by a combination of genes and environment.”

In other words, the most basic equation to understand weight gain is that people gain weight when they consume more calories than they burn — those extra calories get stored as fat. But scientists have found many connections between someone’s risk of weight gain and a wide variety of other factors, including his or her genetic makeup, diet in infancy and childhood, sleep habits, stress levels, and gut bacteria.

In the past decade, there have been a slew of studies suggesting that the calories-in/calories-out theory of weight gain is an oversimplification.

Still, the genetic component of obesity seems to be strong, though researchers have had some false starts as they work out the details. “If both of your parents have obesity, your likelihood of developing obesity is as high as 80%,” Harvard Health notes. That’s just one indication that some people may be born with the odds stacked against them. As with most genetic risk factors, there’s nothing suggesting that someone is destined to be overweight, except in the rarest of circumstances. But for some, the risk is much higher than others.

“Although environmental factors have driven the recent rise in the prevalence of obesity, the heritability of body weight is high,” wrote University of Cambridge scientist Sadaf Farooqi, in a 2011 review of the research. “There is evidence that genetic variation plays a major role in determining the susceptibility to weight gain.”

Still, as Farooqi noted, “environmental factors” are still seen as the primary culprit. People buy more food than they used to and rates of physical activity have gone down. “Searching for the genetic background of excess weight gain in a world of abundance is misleading since the possible biological control is widely overshadowed by the effect of the environment,” argued researchers in a 2010 paper.

What can be done

One thing is certain: For most people, losing weight and keeping it off is hard. One 2013 review of 22 weight-loss studies involving 480,142 people pointed to some interventions that might work but ultimately concluded that “the strength of evidence is low for all strategies.”

All that said, if you are trying to lose weight, don’t despair. A good first step? Talk to your doctor. A primary care physician can help you figure out if there is a medical condition that makes you more susceptible to weight gain. Your doctor can also review the strategies that make the most sense for your lifestyle and health profile, which may involve eating less processed food, consuming smaller portions, trying a low-carb diet, using cognitive behavioural therapy, or working more physical activity into your life. Dietitian-provided weight-loss counseling has also been shown to be effective, so ask your doctor for a referral.

To treat obesity, not just excess weight, some doctors may recommend prescription weight loss drugs or bariatric surgery, though neither work perfectly and both come with risks. Earlier this year, a review of 20 weight-loss studies found that after an initial very-low-calorie diet, prescription drugs, meal replacements, and high-protein diets were the most effective way to keep the weight off.

The simplest advice — eat less, exercise more — isn’t wrong, but for many people, it’s just not that simple.

Not everyone needs to or wants to lose weight. But if excess weight is affecting your health and the simple strategies have not worked, don’t turn to fads or extremes. Instead, bring a doctor or a dietitian into the conversation, and go from there.

This post is part of a continuing series that answers all of your “why” questions related to science. Have your own question? Email [email protected] with the subject line “Q&A”; tweet your question to @BI_Science; or post to our Facebook page.